Control of inductive power transfer pickups

ABSTRACT

Secondary resonant pickup coils ( 102 ) used in loosely coupled inductive power transfer systems, with resonating capacitors ( 902 ) have high Q and could support large circulating currents which may destroy components. A current limit or “safety valve” uses an inductor designed to enter saturation at predetermined resonating currents somewhat above normal working levels. Saturation is immediate and passive. The constant-current characteristic of a loosely coupled, controlled pickup means that if the saturable section is shared by coupling flux and by leakage flux, then on saturation the current source is terminated in the saturated inductor, and little detuning from resonance occurs. Alternatively an external saturable inductor ( 1101, 1102 ) may be introduced within the resonant circuit ( 102  and  902 ), to detune the circuit away from the system frequency. Alternatively DC current may be passed through a winding to increase saturation of a saturable part of a core. As a result, a fail-safe pickup offering a voltage-limited constant-current output is provided.

TECHNICAL FIELD OF THE INVENTION

This invention relates to inductive power transfer; more particularly toloosely coupled systems for inductive power transfer, and in particularthis invention relates to protection means for limiting the amount ofcurrent circulating in a secondary pickup coil of an inductive powertransfer system.

BACKGROUND

The general structure of an inductive power transfer installation isthat there is a primary conductor (or more) energised with alternatingcurrent, and one or more secondary or pickup devices which intercept thechanging flux surrounding the primary conductor and convert this fluxinto electrical energy by means of windings. Often the pickup devicesare mobile, and move alongside, or sometimes (if internal energy storageis available) away from the primary conductors.

There appears to be at least two distinct groups of inductive powertransfer systems amongst the published literature. One group uses a“spread-out transformer” approach for the primary trackway, in which aseries of iron laminations is used along the full length of the trackwayto enhance coupling of the flux to an adjacent set of laminationscomprising the flux concentrating means used to cause the collected fluxto traverse the (sometimes) resonant pickup windings. The energisingfrequency is relatively low (from mains frequency up to about 5 kHz).Often the primary trackway is buried in a road and faces upwards to linkwith pickups beneath a road vehicle that face downwards. This approachprovides tight coupling, and power is received essentially as if itarrives from a constant-voltage source. Examples of this type ofapproach are illustrated in a series of patent specifications fromBolger (e.g. U.S. Pat. No. 4007817 or FIG. 1 of U.S. Pat. No. 4800328).Klontz et al ( U.S. Pat. No. 5157319) describes an alternative tightcoupling, involving a coaxial winding transformer secondary encircling aprimary conductor.

Our group uses as the primary trackway an elongated loop formed from aparallel pair of conductors, without ferri/ferromagnetic material, andflux is coupled through the core (which does include ferri/ferromagneticmaterial) to the windings of the resonant pickup coil. This coupling isdescribed as loose. Some versions of the track are provided with lumpedresonance elements. The delivery of power is controlled by decoupling atthe pickup, using a number of disclosed techniques, and because thesystem uses a resonant circuit as part of the pickup, the current Atproduces appears to come from a constant-current source. The energisingfrequency is relatively high (10-30 kHz), and in some examples theprimary trackway is mounted upon a conveyer rail, facing sideways tocouple with pickups upon self-powered conveyer units, although it is inother instances embedded within a roadway. This type of approach isillustrated in a series of patent specifications from Boys & Green,commencing with WO 92/17929.

For a comparison of these two approaches refer to FIGS. 14 and 15, andalso a conventional transformer (FIGS. 12 and 13), in terms oftransformer equivalent circuits. (All FIGS. 12-17 are prior art). FIG.12 shows an ordinary, tightly coupled transformer having a primarywinding 1201 and a secondary winding 1202. FIG. 13 shows the transformerequivalent circuit, where winding 1301 represents the coupled flux M,winding 1302 represents the leakage flux about the primary, and 1303represents the leakage flux about the secondary. The value of M isobtained from M=k{square root over (L)}₁L₂ where k is typically 95% ormore. FIG. 14 shows a loosely coupled inductive power transfer pickup,with primary conductors 103 and 104, a core 300, and a resonant circuitcomprising inductor 1401 and capacitor 1402. Considering FIG. 15, theequivalent inductance 1504 (M) represents the power coupling (shared)component of the flux while 1503 is the leakage flux (such as the fluxradiated from the core of the pickup while it is carrying a significantresonating current). For loosely coupled systems having a primarypathway in air, the ratio of inductors 1503:1504 is typically 0.7:0.3whereas for the iron-cored primary with iron-cored secondary devices ofBolger and others the ratio is typically more like 0.2:0.8. In FIGS. 15and 17, the trackway (constant-current source 1500 with equivalentinductors 1501, 1502) supplies a constant current. FIG. 16 shows a kindof inductive-power transfer device where winding 1601 is a resonant,controllable winding and 1602 with rectifier 1605 supplies usefulpower—such as a constant current source for battery charging.Considering the transformer equivalent circuit of FIG. 15, the value ofthe short circuit current (if the output was to be shorted) is$I_{sc} = \frac{IM}{L_{1503} + M}$

where M is the inductance of 1504.

We have devised a battery charger which employs loosely coupledinductive power transfer, the subject of patent applicationPCT/NZ97/00053. Considering a practical circuit from the battery chargerin transformer equivalent form, as shown in FIGS. 16 and 17; items 1705and 1706 represent leakage inductance from the actual windings; 1705 forthe large number of turns in the resonating/control winding, and 1706for the power collection winding. The relative proportions of L in FIG.17 are: 1504=30%, 1503=65%, 1705=c. 5%., and 1706=c. 5impedance of theprimary section as seen looking back from the inductors 1503, 1504 (=95%of L) can then be derived by assuming that a short circuit is placed at1708 (dashed line) and is$Z = {\frac{\left( {{5\% L} + X_{c}} \right)95\% L}{{5\% L} + X_{c_{res}} + {95\% L} + {5\% L}} = \infty}$

Since the denominator at resonance is zero then Z is infinite; therebyproviding the basis for stating that the source acts as a currentsource. A Bolger type circuit is equivalent to FIG. 13. The no-loadvoltage will be determined by the output impedance Z=L₁₃₀₃+L₁₃₀₂ whendriven from a voltage source as is done in Bolger type circuits. Theoutput impedance is Z=L₁₃₀₃+L₁₃₀₁ if the circuit were to be driven froma current source.

While the constant-current characteristic of this type of inductivepower transfer system is generally an advantage, it does impose a riskshould a pickup coil enter a state in which there is no control over theamount of current collected. A perfect constant-current source will haveno voltage limit. An uncontrolled current resonating in a resonantsecondary circuit forming part of a loosely coupled inductive powertransfer system may build up to reach high levels if the circuit Q islarge, whereupon a number of adverse results may occur, such ascomponent failure, for example by overheating or breakdown ofsemiconductors or of dielectrics within resonating capacitors and apartfrom loss of function this can lead to the development of fire withinthe pickup device. Our usual methods for controlling secondary currentrely on active control apparatus, actively causing a switching actionabout the resonant secondary when an over-voltage condition is detectedby a voltage comparing circuit. Passive limiting, relying perhaps on theinherent bulk properties of materials should be safer than activecontrol means. Reliance on active control can break down when severalfactors impinge together on a device so that active control becomesleast likely to function when it is most needed. Some systems usingloosely coupled (i. e. constant-current) inductive power transfer havebeen employed in situations where extreme reliability is a desiredfeature. If such systems rely solely on active control to restrict thecirculating current, then in the absence of function by the activecontrol it is likely that a catastrophic breakdown will occur.

Bolger and Ng in U.S. Pat. No. 4,800,328 (Jan. 24 1989) described theapplication of constant-voltage transformer principles to an inductivepower transfer device by providing a saturable pickup core. This is acontrol application. The laminated iron core is intentionally providedwith a saturable site of reduced cross-sectional area. During normaloperating conditions the core is always saturated to a variable extentand the output from the pickup is limited accordingly by the amount offlux remaining within the core. Furthermore, the resonant frequency isdesigned to be less than that of the supply voltage at low loads, sothat as the core moves into saturation, the resonant frequency risestowards the system frequency; coupling improves and more output(resembling a constant voltage) is available. Cores of this type, driveninto saturation will evolve heat from hysteresis losses, and cooling isnot provided for in the region of the constriction, so this approachwould result in a quite temperature-sensitive output voltage. Theinventors have consistently aimed for a constant-voltage approach.

DEFINITIONS

Loosely coupled in relation to the transfer of inductive power meansthat the proportion of flux actually coupling the primary to thesecondary is significantly less than the total magnetic flux present inthe region of the coupling structures.

Ferrimagnetic properties occur in ferrite materials, in which the entireferrite molecule contributes to the magnetic properties. In the mainthese are comparable to ferromagnetic properties; permeability,saturation, hysteresis, etc. occur in ferrimagnetic materials.

Ferromagnetic properties occur in iron, nickel, cobalt, gadolinium, anddysprosium, and their alloys, in which the magnetic properties reside inthe atoms. Useful ferromagnetic materials for this application includepowdered iron, sintered iron, amorphous iron wires, laminations of iron,silicon steel, grain-oriented steel; used alone or in combination.

Saturation is a property of ferri/ferromagnetic materials defined as achange in the permeability of the material as a function of the magneticfield, in which the material exhibits a finite capacity to carry aquantity of flux, so that the permeability falls as the field rises. Ananalogy to saturation is the way that a bath towel can absorb only alimited amount of water, after which the surplus water drips off.

OBJECT

It is an object of this invention to provide an apparatus or a methodfor controlling an inductive power transfer pickup, or at least toprovide the public with a useful choice.

STATEMENT OF INVENTION

In a first broad aspect the invention provides apparatus for controllingan inductive power transfer pickup for use in a loosely coupledinductive power transfer system, said pickup being capable in use ofcollecting power in the form of a current source from a magnetic fluxsurrounding a primary conductor when energised with alternating currentat a system frequency, wherein the pickup includes active control meanscapable of controlling an output voltage or output current, and whereinthe pickup is a resonant circuit which is resonant at the systemfrequency, the pickup includes passive means capable of limiting theamount of a resonating current circulating in said pickup at less than apredetermined maximum amount, said passive means comprising at least onesaturable inductor having a core; at least a portion of the core beingcapable of becoming saturated at a predetermined flux density; thesaturable inductor being connected so as to carry at least a portion ofthe resonating current so that the onset of saturation within the corereduces the effectiveness of the collection of power and so causes theamount of the current entering the resonant circuit to be limited.

In a related aspect the invention provides apparatus as previouslydescribed wherein the core capable of becoming saturated is comprised ofa ferrimagnetic material.

In a related aspect the invention provides apparatus as previouslydescribed wherein the core capable of becoming saturated is comprised ofa ferromagnetic material.

In a related aspect the invention provides apparatus as previouslydescribed wherein the at least one saturable inductor is constructed sothat the saturable portion of the core is shared by both a coupling fluxand by a leakage flux, so that the onset of saturation causes the amountof coupling flux to be diminished and hence the amount of currententering the resonant circuit from the current source is also diminishedand so that the onset of saturation results in a minimal amount ofdetuning.

In another related aspect the invention provides that at least onesaturable inductor is selected to exhibit an onset of saturation withinthe core at or above a selected current so that the onset of saturationwithin the core changes the resonant frequency of the pickup and socauses the tuning of the pickup to deviate from the system frequency,thereby reducing the effectiveness of the collection of power and socausing the amount of the current entering the resonant circuit to bereduced.

In another related aspect the invention provides that at least onesaturable inductor is constructed so that, when in use, the saturableportion of the core is shared by both a coupling flux and by a leakageflux, so that the onset of saturation causes the amount of coupling fluxto be diminished and hence the amount of current entering the resonantcircuit from the current source is also diminished and so that the onsetof saturation results in a minimal amount of detuning.

In a further related aspect the invention includes a core capable ofintercepting the flux; the core having a saturable part having arestricted cross-sectional area capable of exhibiting an onset ofsaturation at a predetermined flux density so that the efficiency ofcoupling between the primary conductor and the pickup circuit is reducedif the material becomes at least partially saturated.

In a yet further related aspect the invention provides that thepredetermined flux density at which the onset of saturation may occur isdetermined by selecting a material having known saturation thresholdproperties from a range of ferrimagnetic or ferromagnetic materials andusing an amount of the selected material within a flux-carrying part ofthe core so that the efficiency of coupling between the primaryconductor and the pickup circuit is reduced if the material becomes atleast partially saturated.

In a related aspect the invention includes a procedure in which theamount of flux required to reach an onset of saturation is controlled bypassing current through one or more additional windings wound over aportion of the core having a predetermined onset of saturation; thewindings being capable of carrying a DC current capable of generating aflux within the saturable portion of the core; the DC current beinggenerated by a controller responsive to power pickup conditions duringuse, so that the efficiency of coupling between the primary conductorand the pickup circuit is thereby controllable.

In a yet further related aspect the invention provides that thesaturable inductor is physically separate from an inductor capable ofintercepting the magnetic flux, and the saturable inductor is connectedwithin the resonant circuit so that the saturable inductor carries atleast a proportion of the total resonating current, and so that theonset of at least partial saturation in the saturable inductor duringuse causes the resonant frequency of the pickup to move away from thesystem frequency.

In a second broad aspect the invention provides a method for operating aresonant inductive power pickup device for an inductive power transfersystem wherein the magnitude of a circulating resonant current withinthe pickup device is capable of being limited so as to remain below anintended magnitude as a result of at least partial saturation beingreached within a saturable core of an inductor included within theresonant circuit of the device, the limiting process being independentof an active control means, so that a voltage limit is provided.

In a third broad aspect the invention provides a method for operating aresonant inductive power pickup device for an inductive power transfersystem, wherein the magnitude of the circulating resonant current withinthe pickup device is controllable as a result of saturation being causedwithin a saturable inductor included within the resonant circuit of thedevice by a magnetising current passed through at least one additionalwinding; the magnetising current being provided by an active controlmeans.

In a fourth broad aspect the invention provides apparatus forcontrolling the amount of power within a power pickup device having asecondary pickup inductor, having a ferromagnetic core, included in aresonant circuit, wherein the apparatus employs a physical property(apart from a permeability greater than that of air at normaltemperature for non-saturating amounts of magnetic flux) of the core,wherein the physical property is deliberately predetermined so that thecore behaves in a manner capable of limiting the pickup of power whenoperating under conditions outside normal use of the pickup.

In a fifth broad aspect the invention provides apparatus for controllingthe amount of power within a power pickup device as describedpreviously, wherein the apparatus includes a ferromagnetic coreincluding in its magnetic circuit at least a portion of materialselected to exhibit a preferably reversible reduction in permeabilitywith a rise in temperature; the permeability reaching substantially 1.0at the Curie point, so that in the event of the core reaching too high atemperature the permeability of the core is reduced, thereby limitingthe voltage circulating within the resonant circuit.

In a sixth broad aspect the invention provides a method for operating aninductive power pickup device for an inductive power transfer systemwherein the output of the pickup device is controlled as a result ofsaturation being reached during normal use in a ferromagnetic pickupcore within the device.

In a related aspect the invention provides a method for operating aninductive power pickup device for an inductive power transfer systemwherein potentially catastrophic circulating resonant currents withinthe pickup device are limited either as a result of saturation beingreached within a ferromagnetic pickup core within the device, so thatthe inductance of the inductor is altered and the amount of powertransferred is reduced.

DESCRIPTION OF FIGURES

The preferred embodiments to be described and illustrated in thisspecification are provided purely by way of example and are in no wayintended to be limiting as to the spirit or the scope of the invention.

FIG. 1: (and section: FIG. 5) shows a simplified diagram of a prior-artresonant pickup according to the invention.

FIG. 2: (and section: FIG. 6) shows a first version of a resonant pickupaccording to the invention.

FIG. 3: (and section: FIG. 7) shows a second version of a resonantpickup according to the invention.

FIG. 4: (and section: FIG. 8) shows a third version of a resonant pickupaccording to the invention.

FIG. 9: shows a simplified circuit diagram for a secondary pickupaccording to the invention, having a predetermined and fixed saturationpoint.

FIG. 10A: shows a simplified circuit diagram for acontrollable-saturation secondary pickup according to the invention.

FIG. 10B: shows an application of FIG. 10 to a dual-cored pickup,providing control of the saturation point of a pickup device.

FIG. 10C: shows an application of FIG. 10 to a single E-cored pickup,also including cancellation of induced currents.

FIG. 11: shows two simplified circuit diagrams for a pickup circuithaving a saturable inductor separated from the pickup inductor.

FIG. 12 with FIG. 13: show a conventional and an equivalent transformercircuit for a prior-art conventional transformer.

FIG. 14 with FIG. 15: show a conventional and an equivalent transformercircuit for a simple inductively coupled resonant pickup

FIG. 16 with FIG. 17: show a conventional and an equivalent transformercircuit for a saturable battery charger having a separate controlwinding, according to the invention.

FIG. 18: is an oscillogram drawn from a sampled waveform display,showing the secondary voltage in a battery charger over a short periodafter starting.

FIG. 19: is an oscillogram drawn from a sampled waveform display,showing the secondary voltage in a battery charger over a longer periodafter starting.

PREFERRED EMBODIMENT.

This invention provides means for limiting the amount of power within apower pickup device having as a pickup a secondary inductor using aferrimagnetic or ferromagnetic core, and forming part of a resonantcircuit for a loosely coupled inductive power transfer system. Theinvention relies on one of the magnetic properties of those types ofmaterial, namely an ability to be saturated. In a loosely coupled,resonant secondary type of inductive power transfer system, largemagnetic fields are more a result of resonant current that circulates inthe high-Q windings 102 during any period when the power being drawnfrom the pickup is less than the amount of power received as magneticflux rather than a direct consequence of the magnetic field collectedfrom the primary conductors. It may be regarded as “voltage-sensitivecontrol of a current source”. Generally this invention provides a“backup to a controller” or a “safety valve” and it is useful to have asafety valve that relies on an intrinsic property of a material ratherthan an active control device such as that described in our publicationWO92/17929, because an active control device may from time to time failfor any one of a number of reasons.

We will describe a protection process relating to start-up conditionsfor which saturation, being a passive control means having no “warmup”or “initialisation procedure”, is well suited.

It is useful to contrast this invention with normal practice. Althoughany ferri/ferromagnetic core will of course reach saturation at somelevel of flux (just as any boiler will blow up if there is no safetyvalve) this invention relates in particular to a method for determiningthe maximum allowable output from a loosely coupled pickup and hence todesigning the windings and core so that the core will saturate at thatmaximum output, and the windings cannot deliver a greater output voltageor current than the predetermined maximum. This is the “safety valve”effect. Different inductive power transfer applications will presentdifferent ratios of usual running output to maximum allowable output anda “stable” application such as a battery charger may have a ratio closerto 1.0 than an application involving power fed to a motorised conveyancewithout storage.

Our preferred active control means use partial decoupling by, in effect,causing a conductive mass to appear within the gap across whichinductive power passes from a primary conductor to a secondaryconductor. The conductive mass is in fact the shorted secondary pickup(see 1604 in FIG. 16). Shorting the secondary winding would in any casebring any output to zero and shorting is a valid action given aconstant-current type of supply. This control means rapidly decreasesthe amount of voltage coming from the secondary pickup and presents a“magnetically reflective” conductive surface to the magnetic fieldsemanating from the primary pathway so that any incident flux induces anequal and opposite flux in the shorted winding. An active controlinvolves an electronic circuit typically including a comparator, adriver for a solid-state switch, and the solid-state switch itself andas such requires a power supply and has a finite “settling” or “warm-up”period which may be a weakness. (See example 1). A loosely coupledpickup circuit controlled in this way resembles a constant-currentsource.

In this specification we use the well-known “E” core as our prototypeexample, although the physical embodiments of inductive power pickupcoils can take many shapes and this invention is applicable to allferri- or ferromagnetic cores. An example of a prior-art ferrimagneticpickup is shown in FIG. 1, wherein an “E” shaped core is shown in faceview at 100, placed in proximity to a pair of primary pathway conductorsshown in section at 103 and 104. This pickup includes a ferrite core101, having a thick central leg 106, around which a pickup coil 102 iswound, and a pair of non-wound limbs 105, 105′. Typically the non-woundlimbs have a lesser cross-section than the central limb 106, because allthe flux passes through the central limb and hence through the coilaround it, while the others each carry only half the flux. In this sheetof drawings, a cross-section of the central core cut at about thesection shown as A-A′, is shown at the right of the corresponding planview. A core of this type in a working example of a resonant inductivepower transfer secondary pickup does not exhibit saturation under usualworking conditions, but if left uncontrolled the circulating (resonant)current may build up to perhaps 20 times the drawn-off current.

There are at least two mechanisms by means of which the effect ofsaturation can be used to destroy coupling and reduce circulatingcurrent. The detuning mechanism acts whenever the onset of saturationcauses the core permeability and hence the secondary inductance todecrease, the resonant frequency of the pickup changes, and decouplingoccurs by detuning from a pre-existing system-wide resonant frequency.The other mechanism relies on terminating the constant-current source ina short circuit when decoupling is required. Refer to the equivalenttransformer circuit of FIG. 15. Reducing (by saturation) the inductanceof the shared portion M (1504) of the equivalent circuit has the effectof terminating the constant-current supply entering through 1501, 1502in a short circuit, whereupon there is no current to pass through 1503and drive the load. This approach to disabling a constant-current supplyby shorting it is preferable to presenting an open circuit, because inthat case the voltage will tend to rise indefinitely—or at least untilcatastrophic failure occurs. The risk of exceeding the ratings ofconductors or components, with consequential breakdown, can be minimisedbecause the maximum output in the absence of control is predictable. Itis determined mainly by the physical properties of the core (e.g. seeFIG. 10) and the number of turns of windings, and so the known maximacan be used when specifying component ratings.

Although these mechanisms may act simultaneously, the detuning effect ofsaturation of the core associated with M (1504) is small. Detuning mayaffect other resonant circuits within an inductive power transfersystem. Thus we prefer to deliberately locate the saturable portion ofthe core underneath the windings and the option in which a differentsaturable material provides the saturable effect is usually to bepreferred.

If on the other hand the saturable portion was in inductor 1503 in FIGS.15 or 17, (representing leakage flux; a saturable collection wing of aflux collecting core) it is clear that only a detuning effect wouldoccur although the constant current input would still seek a sinkthrough the output to the load, which is not as useful as the firstoption. If the saturable element was inserted in the inductor 1706supplying the rectifier, saturation would have almost no effect.

The relationship of current versus voltage for a system where thewindings surround the saturable core section includes a relatively sharpdrop of current with rising voltage in a case where tight couplingexists (see “Illustration” below). Coupled power is proportional to M².In contrast, an alternative method using a separate saturable inductor(see later) results in a more gradual fall of current with risingvoltage because in this case detuning is a dominant factor. That methodhas other advantages.

Illustration

In a battery charging apparatus (as described in applicationPCT/NZ97/00053 in which a controlled high-voltage tuned resonant circuitand a low-voltage circuit which simply provides an output are tightlymutually coupled, the onset of saturation provides a sharp reduction incoupling as shown by the onset of a sharp deviation between prediction(the formula itself not including a saturation term) and measurement asthe drawn current is reduced toward a saturation point at about 210 A.${{Formula}\text{:}\quad V_{D\quad C}} = \left\lbrack {{\frac{2\sqrt{2}}{\pi} \cdot \frac{V_{A\quad C}}{22}} - {0.004I_{D\quad C}} - {0.8\quad V}} \right\rbrack$

where 22 represents the ratio of turns between the coupled windings, VACrefers to the higher voltage of the control windings which are not thewindings responsible for the rectified DC voltage, the 0.004I_(DC)factor represents a leakage flux from wiring and 0.6V represents theforward voltage drop of diodes. The constants in the first term reduceto 0.0409V_(AC).

Table of Results

Measured Measured Calculated Discrepancy; I_(DC) V_(AC) V_(DC) V_(DC)diode drop = 0.6 V 0.6 466 19.5 495.7 210 444 17.0 16.52 0.3 240 41415.4 15.10 0.1 255 361 13.1 12.90 0.0 270 146 4.3 4.09 0.0

Note the abrupt onset of saturation as the drawn-off current falls. (Inthis example the primary current was constant).

Apparatus

Several example design strategies are now described for putting thisinvention into practice, and may be used separately or together. Forinstance, strategy 3 can be applied over strategies 1 or 2 to trim themaximum flux before saturation as an “on-site adjustment” similar tosetting a safety valve, and strategy 3 can even be used as a powercontrol means.

Strategy 1

While retaining the “flux collecting” areas of the pickup core asbefore, provide a reduction of cross-sectional area of the core,preferably at some position within the common flux path. Generally thecommon position would lie inside the windings of the resonant secondaryinductor. See FIGS. 2 and 6. This is a simple approach, althoughproviding a neck or constriction in the core creates a weakened pointthat may lead to breakage. Cooling means may be provided to fill thespace. The first and the second strategies illustrate two ways to reducethe capacity of the core to accommodate a high magnetic flux. FIG. 2shows that the cross-section 203 of the ferromagnetic core isdeliberately reduced from the original cross-sectional area 107—and theactual amount of reduction to provide an upper limit within a particularapplication may be determined empirically or by calculation; including“edge effects” and the like. A non-magnetic, non-conductive spacing 202may be retained in order to locate the core 203 in position and/or toact as a support for the windings 102. In FIG. 2 we have shown the samekind of ferrite in use in the constricted portion as in the remainder ofthe core. However, it could be a different kind of ferrite.

Strategy 2

Provide a different type of ferrite at a position preferably within thecommon flux path; the different ferrite having a more easily saturableproperty. In FIG. 3 we have shown the substitution of a more easilysaturated kind of ferrite material 302 in the common part of the core.The cross-section (FIG. 7) shows that the entire core at this plane ismade of this altered material. This approach has the advantage that apreferred easily saturable ferrite, particularly suitable for thatpurpose, may not be suitable for general use as the entireflux-collecting core. A preferred core material would have a smallhysteresis loop as indicated by the area of a is plotted B-H loopbecause the excursions of B and H are large in a core approachingsaturation, and hysteretic losses are converted to heat. Advantages ofthis strategy are in physical construction and in accurately predictingpickup properties when designing a core. Although one or both legs ofthe core could be modified in this way, modifying the centre retainssymmetry and alters that portion of the core carrying the combinedleakage and coupling flux; including the flux resulting from currents inthe pickup winding 102.

Strategy 3

Using a separate saturable inductor particularly as a de-tuning element,connected elsewhere about the resonant circuit (as shown in FIG. 11) hasthe advantage that the main, flux-collecting core can be optimised forits primary purpose of flux collection and the saturable inductor can beoptimised for its primary control purpose.

Strategy 4

Provide electrical means for saturation of the ferrite, using DC(usually) magnetising currents. FIG. 4 illustrates the principle ofactive saturation level setting means. Here, windings 402 and 403 can,by carrying DC (or AC) currents affect the saturation of the core bybiasing the excursions undergone by the core, as reflected in the B/Hcurve of the core (or a part of the core) towards one limit or theother. (See FIG. 10A, 10B, or 10C for details). This approach is acombination of protection and control. Optionally , the windings may beplaced over more easily saturable zones such as the block 404 of adifferent ferrite, shown for illustration on one side only of the coreof FIG. 4.

Combination Strategies

The fourth or active control strategy can be combined with the previousstrategies so that a purely passive system backs up an active,controllable setting. Such a “fail-safe” approach may be mandatory incertain applications.

It is also possible to include an over-temperature protection, whichamounts to an alternative type of “safety valve” by including a materialwhich saturates particularly easily with rising temperature. In factvirtually all saturable materials are quite temperature-dependent.Furthermore, it may be possible to exploit the Curie point of a selectedmaterial so that at least a portion of the core becomes effectivelynon-ferromagnetic (the permeability tends to 1.0) when raised above thetemperature of its Curie point. Hence overheating of the core or perhapsthe surrounding windings results in a reliable cut off of circulatingpower. Again, it is preferable to place this material beneath thewinding (such as 1504). A disadvantage of this method is the slowresponse time inherent in a process involving heating of a mass;incapable of protection of faster destructive processes such as thoseapplying to excessively reverse biased rectifiers, or tovoltage-stressed capacitors.

Any of these strategies tends to result in a state of saturation or lossof ferromagnetic properties within the core of the pickup duringcircumstances when an amount of power collected by the power pickupdevice from the primary conductors (source of magnetic flux) 103, 104significantly exceeds the amount of power drawn from the power pickup.The point at which saturation occurs can be set by design, materialsselection, and/or by external saturation means (FIGS. 4 and 10A, 10B,10C). Preferably the power pickup device retains in ample capacity thoseportions of the ferromagnetic core which are intended for fluxcollection. In FIGS. 1-4 they are the ends of the three limbs. These maybe extended along the axis of the primary conductors in order to catchmore flux.

FIG. 9 shows a circuit diagram for an inductive pickup according to thisinvention. Here, 103, 104 are primary conductors, and 102 is a secondarycoil in a resonant circuit with capacitor 902 at the system frequency. Abridge rectifier 903 draws power from the resonant circuit, through anexample power conditioner 904, and to a load 905 connected to terminals906 and 907. This circuit relies on saturation of the saturable core300/302 to limit the peak voltage obtainable from the pickup. In effect,the bridge rectifier provides a voltage-limited current source; thevoltage of which depends on the level at which saturation occurs. Theexample power conditioner could for example be a three-terminal linearintegrated circuit which, by series or shunt control, produces aconstant, regulated output voltage at some voltage less than the inputvoltage obtained from across the output of bridge rectifier 903.

FIG. 10A shows another saturable inductive pickup; in this instanceusing a separate winding 1001 on the saturable pickup 300/302 as acontrol means. In this instance, block 1002 is again a controller butits control is exerted by (1) comparing the output voltage against astandard, and if too high (2) passing a greater DC current through means1003 (for preventing induced AC to pass from the inductor 1001 back tothe controller), so that the level at which the core 300/302 becomessaturated is reduced by flux generated by the inductor and the outputvoltage drops. This is a relatively efficient type of control ascompared to a linear series or shunt regulator. This is a form of activecontrol, although it should be designed with a safety valve; the maximumoutput is set at the point at which the saturable core will saturateeven with zero current in windings 1001. In the above, means 1003 isrequired to block an induced high-voltage AC from windings 1001 becausea designer would use a large number of turns in 1001 in order to achievea high flux density without too high a current.

FIGS. 10B and 10C show ways for blocking that high voltage AC bycancellation. In FIG. 10B, the voltage developed in winding 1001 aboutcore 300 is opposed by the voltage of opposite phase developed inwinding 1001′ about a replicated core 300′. 1004 represents connectorsto a control unit such as 1002. The power windings 102 are handled inthe usual way. FIG. 10C shows a different cancellation means, in whicheach of windings 1001, 1001′ develop a steady flux which may cause localsaturation but is more likely to be added within the central leg of thecore 300 beneath the main winding 102, but the AC voltage developed inone control winding is cancelled out by the voltage of opposite phasedeveloped in the other. (103, 104 represent the primary conductors insection). Residual filtering (1003) may still be useful.

FIG. 11 shows two example versions of the invention in which thesaturable inductor 1101 (in the upper version) or 1102 (in the lowerversion) is physically and electrically separated from the pickupinductor, yet is included within the resonant circuit. In theconfigurations shown, the onset of saturation will cause the totalinductance of the resonant circuit (comprising 102, 902, and 1101) tofall and hence the resonant frequency will rise, and hence couplingbetween the track (103, 104) and the pickup will be reduced. The ratioof the inductance of the pickup 102 to the saturable section 1101 shouldpreferably be selected so that saturation is an effective means fordetuning the pickup. The advantages of separating the saturable element1101 from the pickup element 102 include the opportunity of optimisingthe design of the pickup for a given application. Much of the cost andweight of a pickup device resides in the ferrite components of thepickup inductor core and it is useful to be able to optimise the corewith minimal constraints. Also, provision of a saturable element such asa constricted section within the pickup core itself may raise the riskof damage. Furthermore, the saturable element can, if isolated, beselectively cooled with forced air or the like in order to stabilise itsproperties, and in addition the saturable element may be provided withactively driven control windings as illustrated in FIG. 10A. FIG. 11draws attention to the common practice of driving more than one pickupor power consumer from the same primary pathway 103, 104 without adverseinteractions.

Primary Track Considerations

Many of our existing inductive power transfer systems involve the use ofa common system frequency and the primary power supply is run in aself-resonating mode in which the aggregate of resonant frequencies ofthe primary pathway and various loosely coupled secondary circuits setsthe “oscillation” frequency of the power source. A closely similarresonant frequency in all resonant circuits maximises coupling andreduces the risk of frequency hopping or other instabilities. Thesystem-wide resonant frequency is typically 10 to 15 kHz in ourinstallations. If the saturation-based controls used cause detuning, thepower supply used for supplying power to the primary inductive pathwayis preferably a constant-frequency type, because any change of primarysupply frequency could adversely affect other pickups operating alongthe same primary inductive pathway. Furthermore, if the power supplycould “chase” the detuned pickup frequency the control would have noeffect. Hence we prefer to rely on manipulation of the shared inductanceso as to maximise the dependence of M (1504 in FIG. 15) on saturationwhile leaving 1503 unaltered, by careful design of the core.

Secondary Regulation Considerations

The constant-current inductive power transfer pickup of a looselycoupled system becomes, in the presence of saturation-based limiting ofresonant current, a voltage-limited constant-current source. If thesystem was always operated in saturation, it may then become aconstant-voltage source. This is not energy-efficient, for examplebecause of losses in the saturable core, and it is not stable, becauseheating caused by losses causes a drift of saturation.

EXAMPLE 1

In a device using inductive power transfer for battery charging purposesthe saturated-core pickup design has been designed so that the rectifierdiodes are effectively protected from a voltage surge which willotherwise occur at any time that a shorting controller enters the “open”state, or at the initial connection of power. Because a constant-currentsupply exists, the immediate injection of full power results in avoltage surge because consumption rises more slowly. The rate of rise ofcurrent flow from the rectifier to the battery is limited by a seriesinductance.

The design process in respect of saturation comprises the provision of acore which will saturate, when at a low end of an operating temperaturerange, before the output voltage exceeds the peak inverse voltage ratingof the rectifier diodes used. (The “low temperature” is specified onlybecause the flux density at saturation reduces with increasingtemperature). This application uses a bridge comprised of Schottkydiodes each having a peak inverse voltage rating of 45V. The number ofturns is almost 1 in this instance, and the maximum allowable flux istherefore set to be less than an amount giving 90V per turn (the limitfor two diodes in series). In practice we obtain 26V per turn from ourprototype transformer. This application of saturation providesinstantaneous protection and, being based on material properties, doesnot require prior activation of active devices.

In this example, the drawn current is relatively predictable and thevoltage at which saturation occurs may be set to be only a small amountmore than the output voltage. (Other applications such as movingvehicles, having a widely varying load, may require a greater marginbetween output and saturation).

Example measurements are given in FIG. 18 which is a voltage(Y=20V/division) against time (X=c. 150 μs/division) oscillogram drawnfrom a sampled waveform display showing the secondary voltage in abattery charger after starting (time A). We observe that the peaks ofthe first three cycles B, C, D rise in a series, but the fourth peak (E)is only a little greater and the following peaks (F, G, H) are reduced.After the eighth peak (I) the amplitude tends to stabilise. This effectbetween peaks D to I reflects the onset of saturation and limiting ofvoltage amplitude after just 3 cycles of applied power. Had the peaksafter the third peak continued to rise at the same rate, the rectifierwould have been destroyed. (A 12.9 kHz supply; 20V per divisionvertical.)

A second adverse effect is also controlled by the saturable core. Thisis the tendency for the circuit comprising the resonating capacitor andthe series inductor (not the pickup resonant inductor) to form anundamped resonant circuit and undergo repeated large excursions in flow.The rectifier and the battery form part of this circuit yet have littledamping effect. FIG. 19 is a diagram of the waveform envelope of thesecondary output (where A, D, and K are the same points as in FIG. 17)showing that the inclusion of a saturable inductor as the resonantinductor results in effective damping of surges in secondary currentflow.

Variations

Although the examples relate to an “E” shaped core, and although we usethe term “ferrite” in referring to preferred core materials discussedherein, the principles of the invention apply to any configuration andmaterial of any core made of a ferrimagnetic or a ferromagneticmaterial.

In the case of relatively tightly coupled inductive power transfersystems, the manipulation of coupling by saturation is less useful, butdetuning properties may be exploited. The principles described hereinalso apply to situations in which secondary resonance is not normallyexploited and in those, decoupling is the most important mechanism.

Advantages

The proposed secondary current limiting means is passive and relies onthe bulk properties of a material. Therefore the invention should have avery reliable fail-safe control feature. Temperature rises, mechanicalshocks, fractures, and the like reduce the saturation capacity. Onlycooling can increase it. The risk of exceeding any ratings of ancillarycomponents is now brought under control—as in Example 1—regardless ofwhether active protection circuits have been properly activated. Use ofsaturation as a control means, as described herein, is a “wattless” kindof control. If the active section fails, the passive saturation limit isreached.

Finally, it will be appreciated that various alterations andmodifications may be made to the foregoing without departing from thescope of this invention as set forth in the following claims.

What is claimed is:
 1. An apparatus for controlling an inductive powertransfer pickup for use in a loosely coupled inductive power transfersystem, the pickup collecting power in the form of a current source froma magnetic flux surrounding a primary conductor upon energizing theprimary conductor with alternating current at a system frequency,wherein the pickup comprises means for controlling an output voltage oroutput current at a value above a working range of said output voltageor output current, for safety purposes, and wherein the pickup is aresonant circuit which is resonant at the system frequency, wherein thepickup comprises passive means for limiting the amount of a resonatingcurrent circulating in said pickup at less than a predetermined maximumamount, said passive means comprising at least one saturable inductorhaving a core, wherein at least a portion of the at least one saturableinductor exhibits an onset of saturation within the core at or above aselected current so that the onset of saturation within the core changesthe resonant frequency of the pickup and so causes the tuning of thepickup to deviate from the system frequency, thereby reducing theeffectiveness of the collection of power and so causing the amount ofthe current entering the resonant circuit to be reduced.
 2. Theapparatus according to claim 1, wherein the at least one saturableinductor is constructed so that, when in use, the saturable portion ofthe core is shared by both a coupling flux and by a leakage flux, sothat the onset of saturation causes the amount of coupling flux to bediminished and hence the amount of current entering the resonant circuitfrom the current source is also diminished and so that the onset ofsaturation results in a minimal amount of detuning.
 3. The apparatusaccording to claim 2, wherein the core has a saturable part having arestricted cross-sectional area capable of exhibiting an onset ofsaturation at a predetermined flux density so that the efficiency ofcoupling between the primary conductor and the pickup circuit is reducedif the material becomes at least partially saturated.
 4. The apparatusaccording to claim 3, wherein the predetermined flux density at whichthe onset of saturation is determined by selecting a material havingknown saturation threshold properties from a range of ferrimagnetic orferromagnetic materials and using an amount of the selected materialwithin a flux-carrying part of the core so that the efficiency ofcoupling between the primary conductor and the pickup circuit is reducedif the material becomes at least partially saturated.
 5. The apparatusaccording to claim 4, wherein the amount of flux required to reach anonset of saturation is controlled by passing current through one or moreadditional windings wound over a portion of the core having apredetermined onset of saturation; the windings being capable ofcarrying a DC current capable of generating a flux within the saturableportion of the core; the DC current being generated by a controllerresponsive to power pickup conditions during use, so that the efficiencyof coupling between the primary conductor and the pickup circuit isthereby controllable.
 6. The apparatus according to claim 2, wherein thesaturable inductor is physically separate from an inductor capable ofintercepting the magnetic flux, and the saturable inductor is connectedwithin the resonant circuit so that the saturable inductor carries atleast a proportion of the total resonating current, and so that theonset of at least partial saturation in the saturable inductor duringuse cause the resonant frequency of the pickup to move away from thesystem frequency.
 7. The apparatus according to claim 4, wherein theamount of flux required to reach an onset of saturation is controlled bypassing current through one or more additional windings wound over aportion of the core having a predetermined onset of saturation; thewindings being capable of carrying a DC current capable of generating aflux within the saturable portion of the core; the DC current beinggenerated by a controller responsive to power pickup conditions duringuse, so that the efficiency of coupling between the primary conductorand the pickup circuit is thereby controllable.